Memory cells provide data storage for electronic systems.
One type of memory is dynamic random access memory (DRAM). DRAM is dynamically accessible. In DRAM, memory is stored for a short period of time—typically less than a second, and accordingly DRAM requires periodic refreshing. DRAM loses memory when power fails, and accordingly is volatile. DRAM may be accessed and altered quickly.
Another type of memory is static random access memory (SRAM). SRAM may be accessible faster than DRAM, may store memory permanently as long as power is present, and thus may not require refreshing like DRAM. SRAM, like DRAM, loses memory when power is off, and accordingly is also volatile.
While DRAM traditionally utilizes unit cells containing a transistor and a capacitor, SRAM unit cells comprise six transistors, and therefore the cell size for SRAM is much larger than that for DRAM. In high-performance electronic systems, SRAM may be used for instruction storage, while DRAM is used for working memory and short-term data storage. Permanent instructions and data may be stored in DISCs, which are large, bulky, and relatively slow in performance.
Capacitor-less DRAM is becoming increasingly common. In capacitor-less DRAM, charge storage is accomplished without a capacitor, and may, for example, be accomplished utilizing a floating body.
For portable electronics which do not require ultra-high-speed alteration of data and instruction, another type of memory that may be utilized is nonvolatile memory (NVM). NVM is memory which stores data without refreshing, and accordingly memory suitable for storing data for substantial lengths of time (for instance, 10 years) in the absence of power.
NVM unit cells may be random accessible similar to DRAM or SRAM, and may be called NROM or EEPROM when organized in a NOR configuration. An NROM cell may consist of a single transistor unit and have a cell size about half of that of conventional DRAM. NROM may be utilized for storage of instructions.
Another type of NVM may be configured in a NAND configuration, and such may be even smaller than the NROM cell. The NAND configuration NVM may be called NAND-NVM and may be utilized for data storage. NAND-NVM may be accessed serially within a string of 32 cells, and may be slower than NROM.
Memory stored in NVM may be accessed nearly as fast as DRAM or SRAM, and may, for example, be accessed at rates of from about one nanosecond to about 30 nanoseconds. However, writing data or structures into NVM cells may be slower than writing to DRAM or SRAM by many orders of magnitude. For instance, while DRAM cells may be written or erased at from about 10 nanoseconds to about 50 nanoseconds, NVM cells are typically written or erased in the range of from about 100 microseconds to about 10 milliseconds. Additionally, NVM cells may be limited in endurance relative to DRAM or SRAM (in other words, may be limited relative to the number of write/erase cycles that can be performed). Endurance for conventional NVM cells may be about 10 million cycles, while DRAM and SRAM may have nearly infinite endurance (in other words, may have endurance that exceeds the practical lifetime of the electronic system utilizing the DRAM or SRAM). Additionally, NVM cells may require higher voltage to write/erase than DRAM and/or SRAM, with NVM cells utilizing 10 volts to 20 volts, while DRAM and/or SRAM cells may operate at power supply voltage levels of from about 1.5 volts to about 2.5 volts.
The distinction between volatile devices and nonvolatile devices is somewhat subjective. NVM is seldom totally nonvolatile, and accordingly there will be some leakage and loss of data from NVM over long time periods, particularly in environments with substantial heat. Further, a trend is to reduce the amount of refresh required for DRAM in order to reduce power consumption. For the purposes of interpreting this disclosure, NVM means memory that stores data for at least 24 hours at room temperature (about 22° C.) without refresh, and volatile memory, such as DRAM, is memory that requires refresh to store data in time intervals of less than a second during application at room temperature. The storage of data for 24 hours at room temperature is a very minimal tolerance for NVM utilized for distinguishing nonvolatile memory from volatile memory for interpreting this disclosure. In actual applications, NVM may store data without refresh for much longer periods of time, and may, for example, store data for at least about 10 years without refresh.
An example NVM-NAND is flash memory. A flash memory is a type of EEPROM (electrically-erasable programmable read-only memory) that may be erased and reprogrammed in blocks.
Many modern personal computers have BIOS stored on a flash memory chip. Such a BIOS is sometimes called a flash BIOS. Flash memory is also popular in wireless electronic devices because it enables the manufacturer to support new communication protocols as they become standardized, and to provide the ability to remotely upgrade the device for enhanced features.
NOR and NAND are two basic architectures of flash memory. While NOR is random accessible, NAND is currently accessible with strings of 32 bits. Each string may, however, be random accessible. Both NOR-NVM (NROM) and NAND-NVM may be made such that unit cells store single bits or multiple bits. When unit cells are used for single bits of storage, they are referred to as single level cells (SLCs), and when unit cells store multiple bits they are referred to as multilevel cells (MLCs). Manufacturers are transitioning from one bit of storage to two bits of storage per unit cell to double storage capacity. Accordingly, manufacturers are transitioning from SLC-type cells to MLC-type cells. While an SLC device utilizes only two memory states (0 or 1), MLCs may utilize four memory states for storage of two bits of data (00, 01, 10 and 11), eight memory states for three bits of storage, and so on. Accordingly, flash MLC memory cells utilize larger memory windows than flash SLC cells, and differ in programming and addressing requirements.
Both DRAM and NVM offer advantageous and disadvantages. DRAM is fast, but the volatility of DRAM leads to data loss once power is turned off, and the refreshing of DRAM consumes power. NVM is stable in the absence of power, but tends to be slower to program than DRAM. Also, NVM often utilizes higher programming voltages than DRAM, which lowers the endurance of NVM relative to DRAM.
A continuing goal of semiconductor fabrication is to decrease the amount of semiconductor real estate consumed by individual devices, to thereby enable increasing the level of integration of devices.
As the feature size is reduced below 60 nanometers, both volatile memories (for instance, DRAM and SRAM) and nonvolatile memories face various scalability challenges. Scalability challenges common to volatile and nonvolatile memories include short channel effects. Various three-dimensional configurations, thin body substrate constructions (surround gates, finFETs, fully depleted SOI substrates, etc.) have been proposed to address such short channel effects. For conventional DRAM cells, the scalability challenge may also include difficulties associated with reducing the size of the capacitor, and accordingly capacitor-less DRAM has been suggested. For floating gate flash cells, parasitic coupling between adjacent cells during high voltage write/erase operations may also be an issue to be addressed during scaling of the cells. The problem of parasitic coupling may be aggravated by MLC requirements. In order to lower programming voltage, and yet preserve 10 years of charge retention in the absence of power, it has been suggested to replace floating gates of flash devices with floating traps (either SONOS or nanocrystals). These types of devices, especially the nanocrystal embedded charge storage devices, may utilize thinner tunnel insulator in the gate stack, and therefore may utilize lower programming voltages. Replacement of thicker floating gate over-layer insulator (typically oxide or oxide-nitride-oxide) with high dielectric constant insulator (K>10) may further reduce programming voltage.
Additional scalability challenges for flash may be associated with the large memory windows utilized for MLC operation in conjunction with desires to decrease geometry and associated reduction in trapping capacity, threshold dispersion at time-zero due to write/erase cycling, and reliability issues associated with some types of “exotic” dielectric materials being incorporated into the flash devices.
A universal memory may be defined as a memory which addresses functional and application limitations of DRAM, SRAM, NVM and DISC memory storage devices. A universal memory may possess several of, or all of the following attributes: (1) 10 years of memory retention with or without power (nonvolatility); (2) greater than 1013 cycles of endurance; (3) write/erase speed faster than 100 nanoseconds, and in some embodiments in the range of 10 nanoseconds to 100 nanoseconds; (4) write/erase voltage less than four-times the power supply voltage; (5) cell size less than or equal to 4F2 (where “F” is the minimum feature size of the operation utilized to build the cell); and (6) no fundamental scalability limitations. Additionally, it may be desired that the universal memory have low power requirements, and be readily integrated into multi-functional electronic devices and storage systems.
A continuing goal of semiconductor technology is to decrease the amount of semiconductor real estate consumed by multiple types of memory devices, and to thereby reduce costs, power requirements, and interface complexity. Such may enable higher levels of functional integration and new applications. It is thus desired to develop devices, such as universal memory cells and associated structures and processes by which simultaneous advantages of DRAM and NVM (including DISC application) may be realized.